1) Justice in exchange is achieved when the value of the things exchanged is equal.
2) The value of money is not constant through time. This is inflation.
3) Given some inflation, the value of the returned principal is less than the value of the given principal.
4) Therefore it is unjust not to charge over the principal given inflation.
The error is in failing to recognize what is actually exchanged. The argument presupposes that what is exchanged is money now for future money. This is false as future money does not exist and therefore cannot be exchanged.
What is exchanged is some principal for the promise of a return of that principal. The value of that promise is equal to the value of the principal and hence justice is maintained.